1. Field of the Invention
The present invention relates to an improved process for preparing an electrode for electrolysis.
2. Description of the Prior Art
Heretofore, the alkali metal electrolysis such as sodium chloride electrolysis has been mainly carried out by the mercury process. Recently, the pollution of drainage containing mercury component caused by the mercury process has been discussed. The change of the process from the mercury process to diaphragm processes has been required.
The diaphragm processes have been usually worked at higher pH in the electrolysis in comparison with the mercury process. The known electrodes have low oxygen overvoltage. Accordingly, when the known electrodes are used for the diaphragm process or the ion-exchange membrane process about 1 to 3% of oxygen is included in the resulting chlorine, whereby the anode gas can not be directly fed into petrochemical plants etc. It is necessary to use the anode gas after removing oxygen. Accordingly, special equipments and complicated operations are required to increase the cost.
In order to overcome the disadvantages, an electrode which causes a smaller generation of oxygen should be used. The equilibrium potential of oxygen (E.sub.O2) is lower than that of chlorine (E.sub.C12). When an electrode which does not have any selectivity in the electrode reaction of oxygen and chlorine is used, a large amount of oxygen is generated at the potential for generating chlorine.
Thus, in order to reduce the generation of oxygen, it is necessary to provide a coated layer which inhibites an oxygen electrode reaction in the theory of reaction rate.
The selectivity of the electrode for the electrode reaction is called as an electrocatalytic activity which has been estimated by an exchange current density of the electrode material.
It has been known that platinum group metals such as Ru, Pd, Rh, Pt and Ir have such electro-catalysis. The exchange current densities of these platinum group metals on the oxygen gas evolution reaction are as follows. EQU Ru&gt;Ir&gt;Rh&gt;Pd&gt;Pt.
The exchange current densities on the chlorine gas evolution reaction are as follows. EQU Pd&gt;Ru&gt;Ir&gt;Rh&gt;Pt.
From the viewpoints of smaller generation of oxygen and superior electrocatalytic activity on the chlorine gas evolution reaction, palladium is optimum.
However, in the practical use, when palladium is coated in the form of palladium metal, the palladium metal coating is dissolved in the electrolysis and can not be practically used, because of it's corrosive property.
In order to overcome the disadvantages, it has been proposed to use corrosion-resistant electrodes prepared by coating the Pt-Pd alloy on a substrate or by oxidizing the surface of that Pt-Pd alloy. (B.P. 1,147,442, B.P. 1,195,871).
However, the electrocatalytic activity of palladium itself could not be imparted because the alloy of palladium is used and the corrosion-resistance for a long time of the electrode is not satisfactory.
It has been proposed to use an electrode made of Pt-Pd alloy oxide. (B.P. 1,147,442, B.P. 984,973). In order to form the alloy oxide on a titanium substrate, it is necessary to treat it at high temperature in the atmosphere of oxygen under high pressure. In the treatment, the titanium substrate is severly oxidized to be difficult for using it as the electrode. Accordingly, in the proposed method, the Pt-Pd alloy is coated on the titanium substrate and the alloy oxide is formed by the anodic oxidation. The characteristics of the electrode are substantially the same with those of the electrode prepared by oxidizing the surface of the Pt-Pd alloy.
On the other hand, the inventors have studied to coat palladium oxide on a substrate made of titanium etc. However, the adhesiveness of the titanium substrate and the palladium oxide is not enough, and it has not been succeeded.
The inventors have further studied and have succeeded to obtain an electrode which can be practically used, by adding a small amount of the other metal oxide to a large amount of palladium oxide to improve the mechanical strength. However, it has not been succeeded to decrease the consumption of the electrode to substantially zero.
The inventors have further studied the reason why the perfect corrosion-resistance cannot be attained by coating palladium oxide on the titanium substrate and have found that the corrosion is caused by a small amount of metallic palladium. That is, when titanium is directly contacted with palladium oxide or the unreacted palladium compound in the preparation of the palladium oxide coating on the titanium substrate in the thermal decomposing process, the palladium compound is reduced with titanium whereby metallic palladium is formed to contaminate the palladium oxide.
Accordingly, it is considered that the corrosion-resistance is deteriorated by using the electrode having the improved mechanical strength for a long time because the metallic palladium formed by the reduction is dissolved in the electrolysis and the coated layer becomes porous and the coating is fallen down with the generation of the gas from the surface of the electrode.
The inventors have proposed from these facts to prepare an electrode by coating palladium oxide in a form of a complete oxide and a platinum metal on a valve metal substrate made of titanium, tantalum or zirconium.
The feature of the preparation of the electrode is different from a preparation by directly coating a thermally decomposable palladium compound on a substrate and thermally decomposing it, but it is as follows.
Palladium oxide in a complete form is previously formed by thermally decomposing palladium chloride in oxygen or oxidizing palladium black in oxygen. The resulting palladium oxide is dispersed into a butanol solution of a platinum compound which can be thermally decomposed to form platinum metal such as chloroplatinic acid to prepare a slurry for coating. The slurry is coated on a substrate treated by etching in mechanical and chemical manner and baking it at elevated temperature.
In accordance with such preparation, a formation of palladium metal is not found and a thick coated layer having several times of a thickness obtained by the conventional thermal decomposition process can be formed by one coating step and moreover, particles of the coated palladium oxide have larger than those of the conventional thermal decomposition process whereby an improved corrosion characteristic of the electrode is attained. In said case, the platinum component mixed with the palladium oxide should be platinum metal, so that the adhesiveness of the coated palladium oxide with the substrate is improved and the electric contacts between the palladium particles are improved to reduce the electric resistance of the palladium oxide for imparting electrocatalytic activity.
Said electrode has satisfactory electrocatalytic activity and anticorrosive property, however it has a disadvantage that a mechanical peeling-off of the coated layer is easily caused by forming bubbles in the electrolysis.
In order to overcome said disadvantage, the inventors have proposed a preparation of an electrode which comprises forming said coated layer and then, further coating platinum metal as a binder for said coated layer.
The inventors have further studied on these coating processes.